Calculate statistics and update the authorities list page

* Adds a new table for AuthorityStatsSummary
* Adds a rake task to calculate stats per authority as a whole and on a per category basis (based on data already loaded into the database)
* Uses some ruby magic (and a horrendous if statement) to make `percentage_#{attribute}` methods for the stats counts
* Fixes a bug where a single controllers to override in the theme was hardcoded, now loads everything ending in _controller.rb in the theme's controllers directory
* Overrides the authority list controller
* Example wire-up of authority list page - more than happy to take this bit out if it makes implementing #80 harder

Closes #30
Closes #6 
Actually, thinking about it more generally, wouldn't it be better to use the config variable for categories? That way we still get stats for categories that no applications are in yet, which might be helpful to see.